The purpose of this study was to investigate the effect of timed exercise on human circadian rhythms. Subjects were randomized to 5 days of morning or evening treadmill exercise. A measure of internal circadian timing, or phase, was measured at baseline and following the exercise intervention. Change in circadian phase was compared between the morning and evening group.

This study investigated the effect of morning versus evening exercise on internal circadian timing, or phase. Young, sedentary subjects were recruited for this study. Baseline measures included anthropometric, body composition, cardiorespiratory fitness, chronotype, and circadian phase. Subjects were randomized to 5 consecutive days of morning or evening exercise. Thirty minutes of exercise was performed at the designated time and intensity was maintained at a heart rate corresponding with 70% peak VO2. The measure of circadian phase was salivary dim light melatonin onset, indicated by the time when saliva melatonin concentration exceeded 4pg/ml. Post-exercise circadian phase was measured the evening following the last day of exercise. Changes in circadian phase were compared between morning and evening groups.

Inclusion Criteria:

• BMI 18.5-40.0
• sedentary; <=2 hrs structured exercise weekly
• medication free (other than birth control)

Exclusion Criteria:

• diagnosed conditions that would contraindicate safe participation in physical activity
• diagnosed psychiatric conditions
• night or rotating shift work 1-year prior to study
• travel across time zones within previous 4 weeks
• having children <2 years of age
• smoking